The present invention relates to a method of casting molten steel when molten steel is vibrated by the action of an electromagnetic coil. Also the present invention relates to a continuous casting apparatus for the method of casting molten steel and a cast slab which has been cast by the method and the apparatus. More particularly, the present invention relates to a method of casting molten steel, an apparatus for the method of casting molten steel and a cast slab which has been cast by the method and the apparatus, characterized in that: gas and powder trapping caused in molten metal in the process of solidification of the molten metal in a mold can be prevented; cracks on a surface of the cast slab caused when the temperature is not uniform can be prevented; and further the inner structure of the cast slab can be made fine.
As a method for making a solidification structure to be equi-axed crystal so that segregation caused in the process of solidification can be reduced, in continuous casting of steel, electromagnetic stirring is widely used. For example, this technique is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 50-23338. It is possible to obtain an equi-axed structure when molten steel in the proximity of a solidification interface is forcibly given a fluidity by electromagnetic stirring so that prismatic dendrites can be cut apart. In order to enhance an equi-axed crystal ratio, various investigations have been made into the condition of electromagnetic stirring until now and segregation has been somewhat reduced.
However, according to the conventional electromagnetic stirring generated in a mold, an equi-axed crystal ratio by which a sufficiently high quality of product can be produced is not necessarily obtained in the case of producing a type of steel (for example, a type of steel, the carbon content of which is not more than 0.1%) in which it is difficult to form an equi-axed crystal structure. In order to enhance the equi-axed crystal ratio of the above type of steel, in which it is difficult to form an equi-axed crystal structure, it can be considered to increase the thrust of electromagnetic stirring generated in a mold. However, when this method is adopted, a surface velocity of molten steel in the mold is increased, and powder trapping is caused on the surface of molten steel. As a result, a defect is caused on the surface of the product. In some types of steel in which the occurrence of segregation is severely restricted, it is impossible to meet the demand of quality only when the equi-axed crystal ratio is enhanced. In these types of steel, the grain size of the equi-axed crystal structure must be made further fine.
Conventionally, the following technique is reported, for example, the following technique is disclosed in the U.S. Pat. No. 5,722,480. Pulse waves, which are generated by turning on and off an electric current, are given in an alternating static magnetic field so that an electromagnetic force directed to the center of a mold side wall is generated. By this electromagnetic force, a lubricating effect and a soft contacting effect can be provided. However, according to the above method, the electric current is not always made to flow, and an acceleration of the vibrating waves is not controlled. Japanese Unexamined Patent Publication (Kokai) No. 9-182941 discloses a method in which a stirring direction of the electromagnetic stirring is periodically inverted so that a downward flow cannot be developed and diffusion of inclusion to a lower portion can be prevented. However, according to this method, vibrating waves are not given onto the front solidified shell by a shifting magnetic field. Also, it is not a method in which acceleration is controlled so that the solidification structure can be made fine, inclusion can be eliminated for purification and the meniscus can be stabilized.
Further, according to a method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-71557, an electromagnetic coil for generating a magnetic field to rotate molten metal on a horizontal surface is alternated so that it can exist in a static condition. Therefore, a flow velocity of the meniscus is zero in this method. According to a method disclosed in Japanese Examined Patent Publication (Kokoku) No. 3-44858, in order to prevent V-segregation and porosity of a cast slab, in an electromagnetic stirring in which a circulation current is caused on a plane perpendicular to a direction in which a cast slab is drawn out, a stirring direction is inverted at intervals of 10 to 30 seconds. According to a method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 54-125132, the casting temperature is prescribed for preventing ridging of stainless steel and, in order to prevent positive and negative segregation caused in electromagnetic stirring, a ratio of two electric currents, the phases of which are different from each other, is prescribed, and a direction of electric current is switched and an electric current is made to flow in a predetermined direction for 5 to 50 seconds.
Further, according to Japanese Unexamined Patent Publication (Kokai) No. 60-102263, in order to prevent the occurrence of defects caused in casting steel of 9%-Ni which is used for a thick plate at low temperatures, alternating time of electromagnetic stirring is set at 10 to 30 seconds.
In the above techniques, alternating stirring is conducted in a relatively long period. That is, the above techniques are entirely different from a technique in which vibrating waves are given onto the front solidified shell by a shifting magnetic field and acceleration of the vibrating waves is controlled.
Therefore, it is desired to develop a new technique by which the above problems are solved, the solidification structure is made fine, inclusion is purified and further the meniscus is stabilized.
An object of the present invention is to solve the above problems caused in the conventional electromagnetic stirring generated in a mold. That is, it is an object of the present invention to provide a continuous casting method in which vibration is given by a shifting magnetic field so that the equi-axed crystal ratio can be enhanced without the occurrence of surface defect caused by powder trapping and the equi-axed crystal structure itself can be further made fine. Further, it is an object of the present invention to provide a continuous casting apparatus to which the above continuous casting method is applied, and also it is an object of the present invention to provide a cast slab produced by the above method and an apparatus.
It is another object of the present invention to solve problems caused in the casting method in which an electromagnetic force is given to molten metal so that solidification of molten metal can be stabilized and the surface property of a cast slab can be improved.
The summary for the present invention to accomplish the above objects is described as follows.
(1) A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in proximity to a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is alternately given a high intensity and a low intensity of acceleration.
(2) A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal periodically, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is alternately given a high intensity and a low intensity of acceleration.
(3) A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is accelerated by a high intensity and a low intensity of acceleration in a range not exceeding a predetermined flow velocity when the directional vectors of high acceleration and low acceleration in the same direction or in the opposite direction are combined with each other.
(4) A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal periodically in the one direction and the opposite direction, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil.
(5) A method for casting molten metal according to any one of items (1) to (4), wherein a process conducted in the mold is a cooling and solidifying process, and also the process conducted in the mold is a continuous casting process for continuously casting a slab, bloom, slab of medium thickness, or billet.
(6) A method for casting molten metal according to any one of items (1) to (5), wherein a high intensity of acceleration of the vibrating waves in the one direction and the opposite direction is not lower than 10 cm/s2 and a low intensity of acceleration of the vibrating waves in the one direction and the opposite direction is lower than 10 cm/s2.
(7) A method for casting molten metal according to item (6), wherein an acceleration and an acceleration time of the vibrating waves in the one direction, or an acceleration and an acceleration time of the vibrating waves in the opposite direction, and a coefficient of acceleration time (accelerationxc3x97acceleration time) satisfy the following expression.
50 cm/sxe2x89xa6coefficient of acceleration time 
(8) A method for casting molten metal according to item (6), wherein an acceleration and an acceleration time of the vibrating waves in the one direction, or an acceleration and an acceleration time of the vibrating waves in the opposite direction, and a coefficient of acceleration time (accelerationxc3x97acceleration time) satisfy the following expressions.
10 xcex7xe2x89xa6coefficient of acceleration time 
xcex7: viscosity cp of molten metal 
(9) A method for casting molten metal according to item (6), wherein a relation between carbon content C and acceleration satisfies the following expressions.
[C] less than 0.1%:30 cm/s2xe2x89xa6acceleration 
0.1%xe2x89xa6[C] less than 0.35%:xe2x88x9280[C]+38 cm/s2xe2x89xa6acceleration 
0.35%xe2x89xa6[C] less than 0.5%:133.3[C]xe2x88x9236.7 cm/s2xe2x89xa6acceleration 
0.5%xe2x89xa6[C]:30 cm/s2xe2x89xa6acceleration 
(10) A method for casting molten metal according to any one of items (1) to (5), wherein an acceleration stop time or an electric power stop time, the period of which is not more than 0.3 sec and not less than 0.03 sec, is provided in the process of acceleration in the one direction and in the process of acceleration in the opposite direction.
(11) A method for casting molten metal according to item (6), (7), (8) or (9), wherein an acceleration stop time or an electric power stop time, the period of which is not more than 0.3 sec and not less than 0.03 sec, is provided in the process of acceleration in the one direction and also in the process of acceleration in the opposite direction.
(12) A method for casting molten metal according to item (6), (7), (8) or (9), wherein acceleration is generated for t1, subsequently a constant flow velocity is kept for t2, next acceleration is generated in the opposite direction for t3 and thereafter a constant flow velocity is kept for t4 in one period, and molten metal in the mold is periodically vibrated by repeating this period, and a vibration time t1+t2+t3+t4 in one period is determined to be not less than 0.2 sec and less than 10 sec.
(13) A method for casting molten metal according to any one of items (1) to (8) or item (9), wherein the molten metal is periodically vibrated, and a rotating flow in the one direction and the opposite direction is given to the molten metal.
(14) A method for casting molten metal according to item (13), characterized in that: when integration is generated for a certain period of time, the expression of integrated value of (acceleration timexc3x97acceleration) in the one direction greater than integrated value of (acceleration timexc3x97acceleration) in the opposite direction is satisfied; and an average rotating flow velocity caused by the difference between the integrated values is not more than 1 m/s.
(15) A method for casting molten metal according to item (13), wherein acceleration of the molten metal in the mold is conducted for t1, subsequently a constant flow velocity is kept for t2, next acceleration is generated in the opposite direction for t3 and thereafter a constant flow velocity is kept for t4 in one period, molten metal in the mold is periodically vibrated by repeating the period, t1a is a time until the vibrating flow velocity becomes zero in time t1, t1b is a time after the vibrating flow velocity becomes zero in time t1, an expression of t1b+t2 greater than t4+t1a is satisfied, and a rotating flow velocity in one direction caused by the difference in time is not more than 1 m/s.
(16) A method for casting molten metal according to item (13), wherein vibration is periodically given in a period of n cycles, a rotating flow is generated by giving acceleration only in a predetermined direction for the rotating time xcex94Tv after the vibration, and an average rotating flow velocity, number n of cycles and rotating time xcex94Tv satisfy the following expressions.
Average rotating flow velocityxe2x89xa61 m/s 
1xe2x89xa6number n of cyclesxe2x89xa620 
0.1xe2x89xa6rotating time xcex94Tvxe2x89xa65 sec 
(17) A method for casting molten metal according to item (13), wherein a rotating flow is generated by increasing an acceleration in the one direction to be larger than an acceleration in the opposite direction, and an average rotating flow rate is not more than 1 m/s.
(18) A method for casting molten metal according to item (13), wherein an electric current for rotation generating a rotating flow in one direction is further superimposed on an electric current during vibration by an electric current of the electromagnetic coil for generating a shifting magnetic field so that an average rotating flow velocity can be not more than 1 m/s.
(19) A method for casting molten metal according to any one of items (1) to (9), wherein the molten metal is periodically vibrated, and vibration of a short period is further added, and the frequency of the vibration of this short period is not less than 100 Hz and not more than 30 KHz.
(20) A method for casting molten metal according to any one of items (6) to (9), wherein an electromagnetic coil is arranged in the mold or in the proximity of the molten metal pool in the mold when molten metal is poured into and solidified in the mold, the molten metal in the mold is periodically vibrated in the one direction and the opposite direction by a shifting magnetic field generated by the electromagnetic coil, and an electromagnetic brake, which is arranged in a range from the meniscus to a position under the mold distant by 1 m, is applied.
(21) A method for casting molten metal according to item (11), wherein an electromagnetic coil is arranged in proximity to the molten metal pool in the mold when molten metal is poured into and solidified in the mold, the molten metal in the mold is periodically vibrated in the one direction and the opposite direction by a shifting magnetic field generated by the electromagnetic coil, and an electromagnetic brake, which is arranged at a position under the mold distant from the meniscus by 1 m, is applied being synchronized with time at which acceleration of the electromagnetic coil is stopped in the mold or being synchronized with time at which an electric power source is stopped.
(22) A method for casting molten metal according to any one of items (6) to (15), wherein the electromagnetic coil arranged in proximity to the molten metal pool in the mold is arranged in a range under the mold from right below the mold to a position distant from the mold by 10 m.
(23) A method for casting molten metal according to item (22), wherein an electromagnetic brake, which is arranged in a range from a position above the electromagnetic coil distant by 1 m to a position below the electromagnetic coil distant by 1 m, is applied.
(24) A method for casting molten metal according to item (11), wherein the electromagnetic coil arranged in proximity to the molten metal pool in the mold is arranged in a range from a position right below the mold to a position under the mold distant by 10 m, and the electromagnetic brake arranged in a range from the meniscus to a position under the mold distant by 1 m is applied being synchronized with the time at which acceleration of the electromagnetic coil is stopped in the mold or being synchronized with the time at which the electric power source is stopped.
(25) An electromagnetic coil device used for any one of items (1) to (24), comprising: an electromagnetic drive device for periodically vibrating in the one direction and the opposite direction; and a control unit for controlling the electromagnetic drive device.
(26) An electromagnetic coil device used for one of items (1) to (24) comprising; an electromagnetic coil; and an electric power source for supplying an electric current to vibrate the electromagnetic coil periodically in the one direction and the opposite direction or a waveform generating device.
(27) An electromagnetic coil device used for one of items (1) to (24), comprising: an electromagnetic drive device for vibrating molten metal periodically in the one direction and the opposite direction, the electromagnetic drive device having a function of raising an electric current to a command value in the case of changing a vibrating direction; and an electric current control device for controlling the electric current.
(28) An electromagnetic coil device comprising an electromagnetic drive device, a control device for controlling an electric current, and an electromagnetic brake used in any one of items (1) to (24).
(29) A cast slab having a negative segregation zone composed of a multilayer structure, the pitch of which is not more than 2 mm and the number of the layers of which is not less than three, a dendrite or a crystalline structure zone composed of a deflection structure of a multilayer.
(30) A cast slab having a negative segregation zone composed of a multilayer structure, the pitch of which is not more than 2 mm and the number of the layers of which is not less than three, a dendrite or a crystalline structure zone composed of a deflection structure of a multilayer, wherein the thickness of the negative segregation zone, dendrite or crystalline structure zone is not more than 30 mm.
(31) A cast slab characterized in that: a corner point (C) of a central negative segregation line (m) of a negative segregation zone of an average profile of the negative segregation zone of a multilayer structure is determined, or a virtual corner point (Cxe2x80x2) extrapolated from two adjoining sides of a central segregation line (m) of an arcuate negative segregation zone is determined; and parallel lines are drawn from points (E) on two adjoining sides, which are distant from the corner point to the inside of the cast slab by 5 mm, to the two adjoining sides, and a difference between shell thickness D1 at a point of intersection (F) with the central segregation line (m) and shell thickness D2 at the center in the cast slab width direction is not more than 3 mm.
(32) A cast slab characterized in that: a corner point of a center line of dendrite or a crystalline structure zone of deflection structure of a multilayer, which has an average profile thereof, is determined, or a virtual corner point extrapolated from two adjoining sides of a center line of the arcuate dendrite or crystalline structure zone is determined; and parallel lines are drawn from points on the two adjoining sides, which are distant from the corner point to the inside of the cast slab by 5 mm, to two adjoining sides, and a difference between shell thickness D1 at a point of intersection with the central line and shell thickness D2 at the center in the cast slab width direction is not more than 3 mm.
(33) A cast slab characterized in that: a shape of the cast slab is circular; and fluctuation of shell thickness at a point on a central segregation line (m) of a negative segregation zone of an average profile of the negative segregation zone of a multilayer structure is not more than 3 mm.
(34) A cast slab characterized in that: a shape of the cast slab is circular; and fluctuation of shell thickness at a point of a center line of a dendrite or a crystalline structure of an average profile of a dendrite structure or a crystalline structure zone of a deflection structure of a multilayer is not more than 3 mm.
(35) A cast slab provided when molten metal is poured into a mold and solidified while an electromagnetic force is applied to the molten metal by an electromagnetic coil arranged in the proximity of the mold according to item (31) or (33), the cast slab comprising a negative segregation zone composed of a multilayer structure formed in the inner circumferential direction of the mold having pitch P defined by the following expression (2) in a range of D0xc2x115 mm in the thickness direction with respect to solidified shell thickness D0 (mm) at the core center in the casting direction determined by solidified shell thickness D (mm) defined by the following expression (1).
D=k(L/V)n xe2x80x83xe2x80x83(1) 
D: Solidified shell thickness
L: Length from meniscus to core center of electromagnetic coil
V: Rate of casting
k: Coefficient of solidification
n: Constant
P=Uxc3x97t/2 xe2x80x83xe2x80x83(2) 
U: Rate of solidification (dD/dt (mm/s))
t: Period of vibration
(36) A cast slab according to one of items (31) to (35), the cast slab having an equi-axed crystal ratio of not less than 50% on the inside of a negative segregation zone composed of a multilayer structure, on the inside of a dendrite or a crystalline structure zone composed of a multilayer-shaped deflection structure.
(37) A cast slab provided when molten metal is poured into a mold and solidified while an electromagnetic force is given to the molten metal by an electromagnetic coil arranged in the proximity of the mold according to item (32) or (34), the cast slab comprising a dendrite or a crystalline structure zone, the growing direction of which is regularly deflected, having pitch P defined by the following expression (2) in a range of D0xc2x115 mm in the thickness direction with respect to solidified shell thickness D0 (mm) at the core center in the casting direction determined by solidified shell thickness D (mm) defined by the following expression (1).
D=k(L/V)n xe2x80x83xe2x80x83(1) 
D: Solidified shell thickness
L: Length from meniscus to core center of electromagnetic coil
V: Rate of casting
k: Coefficient of solidification
n: Constant
P=Uxc3x97t/2 xe2x80x83xe2x80x83(2) 
U: Rate of solidification (dD/dt (mm/s))
t: Period of vibration